Add MQTT climate precision

[PhilRW]

Description:

Add precision to MQTT climate devices for when home-assistant/frontend#3109 gets fixed.

Pull request with documentation for home-assistant.io (if applicable): home-assistant/home-assistant.io#9896

Example entry for configuration.yaml (if applicable):

climate:
  - platform: mqtt
    name: Study
    modes:
      - "off"
      - "cool"
      - "fan_only"
    swing_modes:
      - "on"
      - "off"
    fan_modes:
      - "high"
      - "medium"
      - "low"
    power_command_topic: "study/ac/power/set"
    mode_command_topic: "study/ac/mode/set"
    temperature_command_topic: "study/ac/temperature/set"
    fan_mode_command_topic: "study/ac/fan/set"
    swing_mode_command_topic: "study/ac/swing/set"
    precision: 1.0

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[ghost]

Hey there @home-assistant/core, mind taking a look at this pull request as its been labeled with a integration (mqtt) you are listed as a codeowner for? Thanks!

This is a automatic comment generated by codeowners-mention to help ensure issues and pull requests are seen by the right people.

[balloob]

Referenced issue is not a bug. Precision is only used for when we're converting between C and F in the backend. target_temperature_step is the property that defines the step size for changing the target temperature.